"This was munitions from the Russian equivalent of our Mechanical and Chemical Industry Corporation being sent to the Syrian Defence Ministry," Erdogan told a news conference on Thursday.

"Their examination is continuing and the necessary will follow," he added.

Turkish jets forced the plane, en route from Moscow to Damascus, to land in Ankara.

Meanwhile, a large bomb exploded in an area housing security and army compounds west of Umayyad Square in the centre of the Syrian capital, according to activisits. There was no confirmation of any casualties.

'Air piracy'

Damascus said the plane had been carrying legitimate cargo and described Turkey's actions as an act of "air piracy", while Moscow accused Ankara of endangering the lives of Russian passengers when it intercepted the jet late on Wednesday.

The grounding of the plane was another sign of Ankara's growing assertiveness towards the crisis in Syria.

Turkey's chief of staff warned on Wednesday his troops would respond "with greater force" if shells from Syria continued to hit Turkish territory.

A spokesperson for Moscow's Vnukovo airport told state news agency Itar-Tass everything put on the plane had cleared customs and security checks and no prohibited items were on board.

Military jets escorted the Damascus-bound Airbus A-320,which was carrying around 30 passengers from Moscow, into Ankara airport late on Wednesday after Turkey received intelligence that it was carrying "non-civilian cargo".

Russia, which has stood behind President Bashar al-Assad's government during an 18-month-old uprising that the opposition says has killed about 30,000 people, angrily demanded an explanation.

"The lives and safety of the passengers were placed under threat", the Russian foreign ministry said in a statement, adding that 17 of its nationals on board were refused access to Russian diplomatic staff.

'Brutal massacres'

Turkey said it would stop more Syrian civilian aircraft using its airspace if necessary and instructed Turkish passenger planes to avoid Syrian airspace, saying it was no longer safe.

"We are determined to control weapons transfers to a regime that carries out such brutal massacres against civilians. It is unacceptable that such a transfer is made using our airspace," Ahmet Davutoglu, Turkey's foreign minister, said.

Turkey has boosted its troop presence along the 900km border and returned fire in response to shelling from northern Syria, where Assad's forces have been battling rebels.

Such approval has in the past been used for strikes against Kurdish bases in northern Iraq. In 2008 Turkey sent 10,000 troops backed by air power over the border.

Syrian refugees fleeing across a river into Turkey spoke of chaos as Syrian government forces battled rebels for control of the area around their home town of Azmarin on Thursday.

Loudspeakers in Azmarin, audible from Hacipasa on the Turkish side, called on rebel fighters to give up.

"Give up your weapons. Come and surrender. We are coming with tanks and planes," they said between bursts of mortar fire.

Risks of deeper involvement

The Syrian opposition is deeply divided. Organisers of a Qatar conference aimed at uniting it said on Thursday it had been postponed until they can agree on fair representation for disparate groups.

US vice-presidential candidates clash on Syria

The rebels are outgunned by government forces but can still strike at will.

"The earlier Bashar goes, the easier the transition in Syria will be," French President Francois Hollande said on Thursday.

"The longer it lasts, the greater the risk of civil war, chaos and partition. I refuse to accept that."

Rebels attacked a Syrian army base near the main northern highway on Thursday to try to consolidate their control over the supply line to Aleppo, days after capturing a strategic town in the area, opposition activists said.

They used at least one tank seized from the army, as well as rocket-propelled grenades and mortar bombs, to hit the Wadi al-Deif base, three kilometres east of the town of Maarat al-Numan, which they captured this week, they said. Assad remarks

Assad described Syria and Turkey as "brothers" in an interview published on Thursday, saying Turkey "has no reason to go to war" over recent cross-border clashes.

"We're having problems along the Turkish border because of the attitude of the Turkish government"

- President Bashar al-Assad

"We should work on this issue together," he told left-leaning Turkish newspaper Aydinlik, after a week of shelling between the neighbours left several people dead.

"In times like this, countries should correct their mistakes by talking to each other. [The] Turkish public is noble. We have no problems with the Turkish people and the Turkish soldiers. Syria is not an enemy to Turkey. We've always known Turkey as brothers."

But the interview, which appeared to be an effort to calm tensions, also carried a rebuke.

"We have problems with the Turkish government," said Assad. "We're having problems along the Turkish border because of the attitude of the Turkish government. [The] Turkish government is also responsible for the deaths. And the reason the relationship has come to this point is also a fault of the Turkish government. Not the Turkish people."